Untested but semi-complete outline. I’m fair at this stuff but beware of cargo culting. Try to understand everything that’s going on if you’re going to use it. Any errors I make become your responsibility. :P

#!/usr/bin/env perl
use strictures;
use CGI ":standard";
use Date::Calc "Today";

my $pid = fork();

if ( $pid )
{
    hold_page();
}
elsif ( $pid == 0 )
{
    write_php();
}
else
{
    die "Couldn't fork\n";
}

exit 0;

sub hold_page {
    print header(),
        start_html("OHAI"),
        h1("Keep panties unbunched!"),
        noscript("It's", (Today())[0] . ", for the love of crabcakes!"
+,
                 "Enable JS!");

    print <<'_HTML_'; # <- single quote matters.
  <script src="https://code.jquery.com/jquery-3.4.1.min.js"
          integrity="sha256-CSXorXvZcTkaix6Yvo6HppcZGetbYMGWSFlBw8HfCJ
+o="
          crossorigin="anonymous"></script>
<script>
function checkPHP() {
  $.ajax({ url: "FILL THIS IN"
          ,timeout: 1000 })
     .done( function( data ) {
        window.location.href = "FILL THIS IN";
     })
     .fail( function( jqXHR, textStatus, errorThrown ) {
         if ( textStatus === "timeout" )
         {
            setTimeout(checkPHP, 2000);
            return; // Keep trying.
         }
         else if ( jqXHR.status == 404 )
         {
            setTimeout(checkPHP, 2000);
            return; // Keep trying.
         }
         else
         {
            alert("WAT? " + jqXHR.status + " " + errorThrown);
         }
     });
}

$(function(){ checkPHP() });
</script>
_HTML_

    print end_html();  
}

sub write_php {
    # Execute script that will write page
    # found at URL "FILL THIS IN"
}
[download]

Thanks - jb

Thanks. I'll give it a look. - jb